The proposed research should provide new information concerning the cellular and subcellular localizations of monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT), the enzymes responsible for the degradation of catecholamines in the rat heart. Previous morphological attempts to localize the enzymes have either been relatively unsuccessful, or have lacked adequate resolution to be definitive. By taking advantage of the pharmacological information that is available concerning the substrate specificities of the enzymes, and by using specific inhibitors of the enzymes, it should be possible to use autoradiographic techniques at the light and electron microscopic levels to determine the cellular and subcellular distribution of the enzymes in the rat heart. Attempts will also be made to utilize immunocytochemical techniques to compare the distributions of the enzymes found with radioactive tracers with those using antibodies coupled to electron dense particles. The proposed research takes on added significance in view of the use of the rat heart as a model for mechanisms involved in the function of the adrenergic nervous system. BIBLIOGRAPHIC REFERENCE: Lowe, M. C., Patterson, M. J. and Reichenbach, D. D.: Lanthanum Penetration into Non-disrupted Myocardial Cells After Isoproterenol Administration. Fed. Proc. Abstract in press (1977).